In prior art air temperature control systems electrical motors are used to drive system elements such as fans, compressors and pumps. System elements such as these are sometimes operated in simple on and off modes. Additionally, sometimes these systems are variably driven according to the thermal load on the air temperature control system. For example, the fan speed may be lowered in some air temperature control systems to deliver less air when ambient conditions require less cooling. In a similar manner, electrical motors driving compressors and pumps may be operated at higher speeds when more cooling is called for in air conditioning systems.
Many air temperature control systems are designed to have a plurality of different temperature control zones in which different amounts of cooling or heating maybe required. For example, the air conditioners of air temperature control systems of office buildings may be required to provide more cooling for offices which face the sun during the afternoon hours than for offices located on the opposite side of the building. Thus, temperature sensors and static pressure sensors are disposed in the various zones of the building in communication with the control systems in order to provide individual control signals representative of the temperature and pressure in the various zones. The control signals provided by the sensors are used by the air temperature control systems to variably control the temperatures of the various zones. This variable control provided by these systems may be performed by varying the speed of the electrical motors which drive the fans, compressors and pumps of the control systems. Therefore, motor control systems are required within air temperature control systems in order to vary the speed of the various motors.
Referring to FIG. 1, there is shown a prior art air temperature control system 10 which illustrates a known method of controlling an electrical motor 14. In the prior art air temperature control system 10 a motor starter 12 receives electrical energy from an electrical energy source, such as an AC supply line. The motor starter 12 selectably provides the received energy at its output by operating either as a substantially open switch or a substantially closed switch. When the motor starter 12 operates as a substantially open switch energy is not provided at its output. When the motor starter 12 operates as a substantially closed switch energy is provided at its output.
The energy at the output of the motor starter 12 of the control system 10 is applied directly to the electrical motor 14. The motor starter 12 may include an electrical contact which is selectably opened and closed in order to control the application of energy to the electrical motor 14. Thus in the motor control method of the air temperature control system 10, the electrical motor 14 may only be either on or off under the control of the motor starter 12. Because the electrical motor 14 of the air temperature control system 10 cannot be operated at any intermediate speeds it is not possible for it to save energy by running the electrical motor 14 at a lower speed.
Referring to FIG. 2, there is shown the prior art air temperature control system 20 which illustrates another known method of controlling electrical motors such as the electrical motor 14. In the prior art air temperature control system 20, the motor starter 12 receives electrical energy from a source such as an AC line and selectably applies the received electrical energy at its output as previously described with respect to the control system 10. However, in the air temperature control system 20 the energy at the output of the motor starter 12 is applied to an inverter 16 rather than to the electrical motor 14. The inverter 16 of the air temperature control system applies varying amounts of the energy it receives from the motor starter 12 to the electrical motor 14. The amount of energy applied to the electrical motor 14 varies according to a variable control signal applied to the inverter 16 by way of the inverter control line 18.
In this manner the electrical motor 14 of the air temperature control system 20 may be operated at variable speeds. This variable speed operation permits energy to be saved when reduced motor speeds are required. However, the presence of the inverter 16 itself introduces some energy loss into the air temperature control system 20. Therefore, when the electrical motor 14 is operated at full speed the air temperature control system 20 is less efficient than the air temperature control system 10 because of the presence of the inverter 16. Thus, the presence of the inverter 16 causes energy inefficiency at high motor speed within the control system 20. It is therefore desirable to provide an air temperature control system permitting the energy efficiency obtained by variable speed operation of the electrical motor 16 as found in the air temperature control system 20, while permitting the efficiency of the air temperature control system 10 when operating at full speed.
The present invention comprises a motor control system for controlling the speed of an electrical motor in an air temperature control system having a control signal determined in accordance with the thermal load of the air temperature control system. An electrical energy source provides electrical energy to the control system. A motor starter has a starter input coupled to the electrical energy source and a starter output coupled to the electrical motor for receiving the electrical energy from the electrical energy source and selectably applying the received electrical energy to the electrical motor. An inverter is directly coupled to the starter input and to the starter output to bypass the motor starter for controlling the speed of the electrical motor in accordance with the control signal. The inverter provides a frequency signal in accordance with the control signal wherein the frequency signal may be a variable frequency signal or a pulse-width modulated signal. The inverter receives AC energy from the energy source and may provide DC energy from the received AC energy wherein the frequency signal may be provided from the DC energy. The electrical motor operates in a high-speed mode and in a reduced-speed mode wherein the mode of operation of the motor is determined in accordance with the control signal. Sensors provide the control signal applied to the inverter. The sensors may be temperature sensors for sensing the temperature to be controlled at various locations or pressure sensors for sensing the pressure at various locations of the system.